(1) Field of the Invention
The present invention relates to a method and apparatus for sensing and measuring vibrational energy in a structure using optical interference patterns.
(2) Description of the Prior Art
There are many structures subject to vibrational energy for which it is necessary to obtain information about the magnitude of the vibrations experienced by that structure. Such structures include space platforms, helicopter blades, mechanical components aboard ships and other vehicles, and mechanical equipment in electrical generators. A variety of analyzers employing optical techniques have been developed to measure the vibrational energy in the structures.
One such analyzer is illustrated in U.S. Pat. No. 3,449,944 to Zavodny. The analyzer shown therein includes a source of coherent electromagnetic radiation such as a laser and a lens mounted to a structure experiencing vibrational energy. A plurality of spaced beams or rays of electromagnetic radiation developed by the source are directed through the lens, which preferably has non-parallel faces and which is capable of causing the rays to intersect each other at a location spaced from the lens. The intersecting rays of coherent radiation form a phase interference pattern which varies markedly and in a predictable fashion as the lens is moved in the slightest degree as a result of the vibration being imparted thereto. The analyzer also includes a detector which responds to changes in the interference pattern.
U.S. Pat. No. 4,525,626 to Kush et al. illustrates another analyzer for measuring the dynamic behavior of a structure. The analyzer comprises a single optical fiber attached to or imbedded in the structure in a predetermined pattern relative to the beams longitudinal dimensions and the characteristic resonant vibration bending modes of the structure. The pattern is arranged to prevent cancellation of optical signal phase variations induced by even order structural vibration modes. A light beam is launched at the input end of the fiber to propagate therethrough in at least two modes. These modes form interference patterns, at the end of the fiber, with intensities that vary as functions of the vibration modes of the structure. The intensity modulated light is coupled to a light detector wherefrom electrical signals are provided that are representative of vibration modal responses.
U.S. Pat. No. 4,577,508 to Chaplin illustrates still another optical vibration analyzer. The Chaplin device has a fiber optic probe which collects back scattered light from a vibrating surface illuminated with non-coherent light. The probe is positioned close to the vibrating surface such that the amount of light scattered back into the fiber optic probe varies with the instantaneous relative separation between the probe and the vibrating surface. A photo-diode detects the varying average intensity of light received by the probe and generates a light intensity signal which is amplified to a usable level. Vibrations in the observed surface are excited by a sweep oscillator and the received signal is processed through a tracking filter locked to the sweep oscillator. The filtered output is monitored and varies directly with the vibration amplitude of the surface.
U.S. Pat. No. 4,801,799 to Tromborg et al. relates to a fiber optic vibration sensor for detecting vibrations in a reflecting surface located externally of the sensor. The sensor includes a transparent body, source and return optic fibers fastened to the body, and a light collimating lens which forms light emanating from the source fiber into a column which extends in a predetermined viewing direction relative to the body. The sensor is positioned relative to the reflecting surface so that the column strikes the reflecting surface and is reflected back through the lens to a focal point in the vicinity of the end of the return optic fiber.
Similar devices employing optical techniques are used to detect acoustic vibrations. U.S. Pat. Nos. 4,446,543 to McLandrich et al. and 4,599,711 to Cuomo illustrate two such devices.
The McLandrich et al. patent relates to a hydrophone which remotely senses impinging acoustic energy. Light output from a laser is split and launched into a single mode fiber which transmits it to an optical resonator at its end. The resonator is made up of a fixed fiber end having a dielectric coating and a movable mirror which is displaced in response to an incident acoustic signal. The acoustic signal displaces the mirror so that mirror movement modulates the intensity of the reflected beam which is transmitted back through the single mode fiber and received at a detector. The frequency and magnitude of the reflected beam yield acoustic signal information.
The Cuomo patent illustrates a transducer having one light transmitting fiber and two receiving fibers having different core diameters. The two receiving fibers are separated at one end and combined at a common distal end in the vicinity of a reflective surface parallel to the fiber end plane which is sensitive to axial motion caused by minute pressure changes such that any displacement of the reflector from equilibrium will increase or decrease the illuminated areas of the two receiving fibers which can be used to generate a processed output signal proportional to this motion.
Piezoelectric transducers are also used to measure vibrational energy in structures. These transducers however are large in size, have nominal receive sensitivity relative to size and require complex wiring and digital equipment to analyze the recorded vibrational energy. They also require complex shielding and construction techniques to reduce electromagnetic interference.
Accordingly, it is an object of the present invention to provide an apparatus for measuring vibrational energy in a structure which is smaller in size and easier to use.
It is a further object of the present invention to provide an apparatus as above which does not require complex computational devices.
It is still a further object of the present invention to provide an improved method for sensing and measuring vibrational energy in a structure.
Still other objects and advantages will become more apparent from the following description and drawings wherein like reference numerals depict like elements.